<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>动画菜单</title>
<style type="text/css">
* { 
	margin: 0; 
	padding: 0; 
	font-size: 14px;
}
a { 
	color: #333; 
	text-decoration: none; 
}
ul{ 
	list-style: none; 
}
.nav {
	height: 30px; 
	border-bottom: 5px solid #F60; 
	margin-left:50px; 
	width:600px;
}
.nav li { 
	float: left; 
	position:relative; 
	height:30px; 
	width:120px 
}
.nav li a { 
	display: block; 
	height: 30px; 
	text-align: center; 
	line-height: 30px; 
	width:120px; 
	background: #efefef; 
	margin-left: 1px; 
}
.subNav li a{ 
	background:#ddd 
}
.subNav li a:hover{ 
	background:#efefef
}

/*css方法一*/
/*.subNav{ position:absolute; top:30px; left:0; width:120px; display: none;}*/
/*.nav li:hover>.subNav{
	display: block;
}*/
/*css方法二*/
.subNav{
	position: absolute;
	top: 30px;
	left: 0;
	width: 120px;
	height: 0;
	overflow: hidden;
}
ul>li ul{transition:height 0.5s;}
ul>li:hover ul{height: 120px;}




</style>


<!-- 用js的方法 -->
<script>
// window.onload=function(){
//     var aLi=document.getElementsByTagName('li');
// 	for(var i=0; i<aLi.length; i++){
// 		aLi[i].onmouseover=function(){
//             //鼠标经过一级菜单，二级菜单动画下拉显示出来
	        
// 	        var This = this.getElementsByTagName('ul')[0];
// 	        clearInterval(This.time);
// 	        This.time=setInterval(function(){
// 	            This.style.height=This.offsetHeight+20+"px";
// 	            if(This.offsetHeight>=120)
// 	                clearInterval(This.time);
// 	        },30)
	        
    
    
// 		}
//         //鼠标离开菜单，二级菜单动画收缩起来。		
// 		aLi[i].onmouseout=function(){
// 			var This = this.getElementsByTagName('ul')[0];
// 			clearInterval(This.time);
// 			This.time=setInterval(function(){
// 			    This.style.height = This.offsetHeight-20+"px";
// 			    if(This.offsetHeight<=10){
// 			        clearInterval(This.time);
// 			    }
// 			},30)
// 		}
// 	}
// }
</script>
</head>
<body>
<ul class="nav">
    <li><a href="#">一级菜单</a>
    	<ul class="subNav">
        	<li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
        </ul>
    </li>
    <li><a href="#">一级菜单</a>
    	<ul class="subNav">
        	<li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
            <li><a href="#">二级菜单</a></li>
        </ul>
    </li>
    <li><a href="#">一级菜单</a></li>
    <li><a href="#">一级菜单</a></li>
    <li><a href="#">一级菜单</a></li>
</ul>
</body>
</html>